1. Field of the Invention
The present invention relates to a motor control device of an electrically driven motor and, more particularly, to a control device of a type used to drive a vehicle wheel in an electric automobile such as, for example, a battery driven automobile, a fuel battery driven automobile or a hybrid automobile with an engine concurrently.
2. Description of Related Art
In the electric automobiles, a synchronous motor and an induction motor are generally employed and are driven by a current, which has been converted from a direct current of a battery into an alternating current by an inverter. Since the inverter, comprised mainly of a plurality of semiconductor switching elements, is supplied with a large amount of electric current for the motor drive, heat generation is considerable. Since the semiconductor switching element has its characteristic variable with temperature and may be apt to be impaired by an excessive heating, the inverter is generally provided with a cooling unit. It is to be noted that in the conventional in-wheel motor drive device, in order to secure the reliability, a wheel support bearing unit, a speed reducer or a reduction gear, the motor or the like are measured as to their temperature to monitor an excessive loading so that in dependence on the measured temperature the driving current of the motor is limited or the motor rotation number is reduced. In this respect, see, for example, the patent document 1 listed below.
[Patent Document 1] JP Laid-open Patent Publication No. 2008-168790
As discussed above, the inverter employed in the conventional electric automobile is provided with the cooling unit and is, therefore, prevented from an excessive increase of the temperature with a normal drive. It has however been found that when on a sloping road or the like the automobile is driven under a high torque generating condition for a long time, the flowing electric current then flowing becomes large, and so it is often that change in characteristic or impairment of the inverter occurs as a result of the excessive heating. Also, the excessive increase of the temperature of the semiconductor switching elements may possibly lower the life of the inverter. Thus, the change in characteristic of the inverter, impairment thereof and the reduction in life of the inverter leads to a change in controlling characteristic of the motor drive and the incapability of motor drive. In addition, where the driving current of the motor is restricted by monitoring the excessive loading through the measurement of the inverter temperature, the drivability of the automobile is possibly disturbed suddenly.
In view of the foregoing, the present invention has for its object to provide a motor control device of a motor which is effective to prevent the inverter from being changed in characteristic, being impaired and/or lowering the life, and also to accomplish a proper countermeasure to be taken quickly without suddenly disturbing of the drivability of the automobile.
Hereinafter, the summary of the present invention will be hereinafter described with the aid of reference numerals shown in the accompanying drawings and used for facilitating a better understanding of preferred embodiments of the present invention.